Add pretrained DNSM models

What's Changed

• Pretrained DNSM models can now be downloaded and loaded with pretrained.load (Issue #161) by @matsen in #162
• Implement Whichmut Trainer and Loss Function (Issue #152) by @jgallowa07 in #157
• model references by @matsen in #164
• Add masking support to ParentIndependentBinarySelectionModel by @jgallowa07 in #166
• Release pretrained DASM model with associated demo notebook by @matsen in #168

Full Changelog: v0.2.3...v0.2.4